for the coefficent - LCM multiply the coefficient - what is the smallest number
that both 14 and 21 divide evenly.
So look at x^3 and x^5; Do you see that x^5 is the smallest monomial that
both x^3 and x^5 divide evenly?
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And looking at b^7 and b^2; Do you see that b^7 is the smallest?